This application is related to my co-pending application, Ser. No. 144,991 now U.S. Pat. No. 4,300,397.
This invention relates to the field of roof tension bolts used in underground mines, and was developed to circumvent a disadvantage which is widely experienced in the application of mechanically anchored roof bolts in underground mining.
It is noted that roof bolting is used as the primary means of roof support in underground mines using the room and pillar mining method, which comprises 90% of U.S. coal mined underground. Typically, four to six foot deep holes are drilled vertically in the overlying rock strata. These holes are normally 1 to 13/8 inch in diameter and spaced on a four foot square grid. Steel rock bolts, or roof bolts, are inserted in these holes and either grouted in the hole for essentially their full length or provided with an expanding anchor at the upper end. A roof plate and bolt head are provided at the lower end.
In the latter case, the bolt is normally tensioned to one half of the yield strength of the bolt, as provided in the Code of Federal Regulations, Title 30, Part 75.200. The tensioned connection thus formed between the rock which houses the anchor and the roof surface at the other end of the bolt renders the roof structure much more competent and self supporting. The disadvantage in practice is that the tension in the roof bolt, which is established at the time of installation, gradually decays with time. The rate of bolt tension decay is frequently highly variable from bolt to bolt, so that a population of bolts initially installed with perfectly uniform tension frequently displays a tension variability characterized by a standard deviation of 20% of the mean instantaneous bolt tension within a very few hours after installation. For example, a population of bolts which are all installed with a tension of 7000 lbs. may within one hour after installation include bolts on which the tension has decayed to 3000 lbs., as well as bolts which still retain 6500 lbs. tension and a variety of intermediate tension values. The example cited is typical and the associated bolt tension variability which thus grows with time, materially detracts from the quality of the roof support and hence from safety in the mine. To data to method or means has been available or known to deal with the basic problem just described.
This invention is accordingly directed to a novel method and novel means for reducing the time dependent variability in roof bolt tension.
A principal object of the invention is to provide a practical and convenient method and means of decreasing time dependent variability of bolt tension in a population of roof bolts, where the gradual loss of bolt tension is unavoidable in itself. An additional object of the invention is to achieve a substantial reduction in bolt tension variability with time in a population of roof bolts, using a modification and augmentation of the equipment means already in use to drill the roof bolt hole and tension the roof bolt.
A further object of the invention is to create a method for monitoring the behavior of the roof in a particular section of a mine by monitoring deviations from established bolt tension decay rates, since such deviations are indicative of movements of the mine roof.